Characterization of a homogentisate dioxygenase mutant in Pseudomonas chlororaphis O6

Transposon mutagenesis of Pseudomonas chlororaphis O6 was performed with the transposon Tn5 to investigate genes involved in production of secondary metabolites. A mutant, termed Org, produced intense dark-brown pigmentation on rich medium. The Tn5-flanking sequence of the Org mutant showed high homology with the hmgA gene encoding the enzyme homogentisate dioxygenase, involved in the degradation of aromatic amino acids such as tyrosine. Growth of the hmgA mutant on L-tyrosine as sole carbon and energy sources was impaired. Growth on L-tyrosine was restored and production of the brown melanin pigment was eliminated when the mutant was complemented with the wild-type hmgA gene. The change in aromatic amino acids metabolism caused by the deletion of the hmgA gene function did not impair production of phenazines and biological traits connected to these secondary compounds: inhibition of fungal growth and inhibition of barley seed germination.